This invention relates to pregelatinized non-granular starches and flours that are inhibited and to a process for their preparation.
Native starch granules are insoluble in cold water. When native granules are dispersed in water and heated, however, they become hydrated and swell. With continued heating, shear, or conditions of extreme pH, the gelatinized granules fragment and the starch molecules are dispersed in the water, i.e., solubilized.
Pregelatinized starches (i.e., cold-water-soluble or dispersible starches) are typically prepared by thermal, chemical, or mechanical gelatinization. The term "gelatinized" or "cooked" starch refers to swollen starch granules which have lost their polarization crosses and which may or may not have lost their granular structure.
The thermal processes generally used to prepare such starches include batch cooking, autoclaving, and continuous cooking processes in a heat exchanger or jet-cooker. The thermal dispersion of a granular starch in water involves a complex mechanism. See the discussion at pp. 427-444 in Chapter 12, by Kruger & Murray of Rheology & Texture in Food Quality, Edited by T. M. DeMan, P. W. Voisey, V. F. Rasper, and D. W. Stanley (AVI Publishing, Westport, Conn. 1976), at pp. 449-520 in Chapter 21 of Starch: Chemistry & Technology, Vol. 2, edited by R. Whistler (Academic Press, New York, N.Y. 1967) and at pp. 165-171 in Chapter 4 by E. M. Osman of Food Theory and Applications, edited by P. C. Paul and H. H. Palmer (John Wiley & Sons, Inc., New York, N.Y. 1972). The process begins at the gelatinization temperature, as water is absorbed into the starch granules, and continues as the hydrated granules swell and disrupt into smaller granular fragments until the starch finally approaches a molecular dispersion. The viscosity of the cook changes significantly during this process, increasing as the granules hydrate and swell and decreasing as the granular fragments are reduced in size. An appropriate amount of shear aids in breaking down the swollen granular fragments to give a molecular dispersion without substantial molecular degradation.
Depending on the starch base, the pregelatinized starch will exhibit specific texture and viscosity characteristics after the starch is dispersed in water. Starches containing amylose will exhibit a gel-like non-cohesive texture. Starches containing high levels of amylose, for example, over 40%, will set to a very firm gel. Unmodified amylose-containing starches pregelatinized by drum drying or extrusion frequently have a pulpy texture when dispersed in water. Starches which contain mainly amylopectin, i.e., waxy starches, do not provide the same gel characteristics as amylose-containing starches. The dispersions of unmodified pregelatinized amylopectin-containing starches exhibit a cohesive and runny texture when dispersed in water.
The texture can be improved if the waxy starches are chemically crosslinked prior to pregelatinization. The crosslinks reinforce the associative hydrogen bonds holding the granules together, inhibit the swelling and hydration of the starch granules during pregelatinization, and consequently, the crosslinked starch granules remain intact. When pregelatinized powders of the chemically crosslinked starches are dispersed in water, the dispersions have a non-cohesive and salve-like texture, which is described as heavy or short.
It is desirable for a pregelatinized starch to be bland in flavor. Many starches such as corn, sorghum, and wheat contain small quantities of unsaturated fatty acids. The fatty acids may develop rancid flavors due to air oxidation. In addition, the proteins present give the starches an undesirable cereal taste. Certain starches, such as corn and waxy maize, are not used in thickened food compositions due to "woody" or "popsicle stick" off-flavors resulting from pregelatinization. See U.S. Pat. No. 4,303,451 (issued Dec. 1, 1981 to W. C. Seidel) which discloses a method for preventing the development of "woody" off-flavors in pregelatinized waxy maize starches. The starch granules are heated, prior to gelatinization, at about 120-200.degree. C. for 0.1-24 hours. The heating time must be insufficient to effect dextrinization but sufficient to prevent formation of woody off-flavors during pregelatinization. The texture and flavor of corn, wheat, rice and sago were modified by this heat treatment, but these starches gave inconsistent and non-reproducible results in food compositions (see Col. 2, lines 14-18).
In some applications, chemically modified starches and flours are unacceptable or undesirable. Thus, there is a need for unmodified pregelatinized non-granular starches which have the textural properties of chemically crosslinked pregelatinized non-granular starches and which are substantially free of off tastes.